1. Field of the Invention
The present invention relates to an antenna device and a radio apparatus, and in particular to an antenna device having multiple resonant frequencies and a radio apparatus equipped with the antenna device.
2. Description of the Related Art
There is a trend that mobile phones or personal computers (PCs) with radio capability have multiple purposes and multiple functions. The above trend requires an antenna device configured to work in multiple frequency bands or in a broad frequency range.
In order to meet such a requirement, antenna devices having multiple resonant frequencies or a broad frequency range are disclosed in Japanese Patent Publication of Unexamined Applications (Kokai), No. 2007-202085 or No. 2005-191718.
The broadband antenna of a built-in type disclosed in JP 2007-202085 is formed by a narrow strip shaped antenna element having an arc shaped portion facing a ground conductor and a projection on a back of the arc for adjusting impedance. An end of the narrow strip shaped antenna element is connected to the ground conductor.
The antenna disclosed in JP 2005-191718 is formed by triple layered and fan shaped conductive patterns having a portion corresponding to a pivot of the fan and facing a nearby ground conductor. According to a disclosed example, the antenna have resonant frequencies in a 3.7 gigahertz (GHz) band and a 6.2 GHz band, and may extend a frequency characteristic up to a higher frequency band.
The broadband antenna disclosed in JP 2007-202085 includes the element having a core portion having an end being short-circuited with a ground plate. The antenna has the arc shaped portion on a side of the element facing the ground plate, and has the projection for adjusting the impedance on the back side. As shown in FIG. 2 of JP 2007-202085, consequently, the antenna may possibly have a problem that a size of the antenna in a direction perpendicular to a side of the ground plate is likely to increase. Such a problem is obvious, e.g., in a note type personal computer (note PC), e.g., having a broadband antenna just above a display.
The antenna disclosed in JP 2005-191718 is formed in such a way that an arc of the fan sticks out in a direction perpendicular to a side of the ground conductor of a dielectric substrate. The antenna may possibly have a problem that a size of the antenna in a direction perpendicular to the side of the ground conductor is likely to increase. Such a problem is obvious, e.g., in a note PC having a broadband antenna just above a display. The antenna may possibly have another problem that the antenna needs to be somewhat thick due to the triple-layered structure, and thus the layers may possibly need to be aligned with one another.
If forcibly given insufficient size in the direction perpendicular to the side of the ground conductor, the above antennas may possibly suffer from a mismatch caused by decrease in impedance as observed at feed portions.